The present invention relates to interleavers used in coded modulation methods and, more particularly, the use of interleavers in fading channel applications, e.g., digital cellular mobile radio.
For some communication channels, coded modulation methods have been found to provide a "coding gain" in signal power (compared to so-called "uncoded" modulation methods) with the result of improving the error performance of the system without requiring additional bandwidth. For example, trellis coded modulation has proven to be a practical power-efficient and bandwidth-efficient modulation method for "random-error" channels characterized by additive white Gaussian noise (AWGN). This method is now being widely used in commercial telephone-line modems and has resulted in an increase of line rates of those modems to as much as 19.2 Kbits/s.
Those in the art, as in the above-mentioned co-pending parent application, are now investigating the applicability of trellis and block coded modulation methods to "burst-error" channels, such as, "fading" channels, i.e., channels in which the signal amplitude can become so weak that accurate recovery of the transmitted information is difficult. These fading channels can be found, for example, in the digital cellular mobile radio environment (hereafter referred to more simply as "mobile radio"). In applying a block or trellis coded modulation method to mobile radio, it is desirable that the code exhibit a property called "time diversity" to improve the error performance of the communication system. This time diversity is manifested by an interdependence between the signal points that are produced by the coded modulation method. For example, consider the case where two signal points are produced over a time interval and there is a time-diverse interdependence between these signal points. As a result of this interdependence, the input data represented by the two signal points may be able to be accurately recovered even if one of the transmitted signal points is lost. However, the improvement in the error performance due to this time-diverse interdependence is limited in the mobile radio environment because typically a burst error extends over a number of time adjacent signal points. Continuing the above example, if both time-diverse transmitted signal points were lost due to a burst error, the input data may not be able to be accurately recovered. As such, the interdependence of signal points alone cannot be relied on to permit accurate recovery of the original signal. Rather, in such environments, an interleaver is often used to separate the interdependent (time-diverse) points to reduce the effects of the fading channel and further improve the error immunity of the system.
In accordance with known interleaver design, an interleaver collects, or frames, the signal points to be transmitted into a matrix, where the matrix is sequentially filled up a row at a time. After a predefined number of signal points have been framed, the interleaver is emptied by sequentially reading out the columns of the matrix for transmission. As a result, signal points in the same row of the matrix that were near each other in the original signal point flow are separated by a number of signal points equal to the number of rows in the matrix. Ideally, in design of the interleaver, the number of columns and rows would be picked such that interdependent signal points, after transmission, would be separated by more than the expected length of the error burst for the channel. However, this may not be practicable, for as the number and rows are increased, so is the signal delay due to the framing of the signal points. As a result, there is a system constraint on the size of the interleaver in order to keep the signal delay within acceptable limits. On the other hand, constraining the size of the interleaver limits the separation of those time-diverse interdependent signal points and, as a result, the improvement in error performance due to the interleaver.